U.S. patent number 5,654,686 [Application Number 08/507,807] was granted by the patent office on 1997-08-05 for electronic vehicle theft detection system employing a magnetic field sensor.
This patent grant is currently assigned to Prince Corporation. Invention is credited to James R. Geschke, Thomas R. Olson.
United States Patent |
5,654,686 |
Geschke , et al. |
August 5, 1997 |
**Please see images for:
( Certificate of Correction ) ** |
Electronic vehicle theft detection system employing a magnetic
field sensor
Abstract
An electronic vehicle theft detection system employing a
magnetic field sensor for mounting within a vehicle to provide
signal information representing the earth's magnetic field. The
sensor is coupled to an electrical processing circuit for sampling
the data provided by the sensor and for generating a theft alarm
signal when the heading of the vehicle has changed due to
unauthorized movement of the vehicle.
Inventors: |
Geschke; James R. (Holland,
MI), Olson; Thomas R. (Holland, MI) |
Assignee: |
Prince Corporation (Holland,
MI)
|
Family
ID: |
24020217 |
Appl.
No.: |
08/507,807 |
Filed: |
July 26, 1995 |
Current U.S.
Class: |
340/426.26;
340/425.5; 340/988; 701/530 |
Current CPC
Class: |
B60R
25/1004 (20130101) |
Current International
Class: |
B60R
25/10 (20060101); B60R 025/10 () |
Field of
Search: |
;340/426,423.5,988,992,994 ;364/424.01,424.02,443,457 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Crosland; Donnie L.
Attorney, Agent or Firm: Price, Heneveld, Cooper, DeWitt
& Litton
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. An electronic theft detection system for a vehicle
comprising:
a magnetic field sensor for detecting the earth's magnetic field
and for providing first electrical sensor signals representing a
fixed direction of the vehicle in relation to the earth's magnetic
field when parked;
a processing circuit coupled to said sensor for processing signals
from said sensor, said processing circuit responsive to said first
sensor signals for calculating and storing fixed direction
information representing the heading of the vehicle when parked,
said processing circuit comparing said fixed direction information
with signals from said sensor representing current heading
information for the vehicle and generating an alarm output signal
indicating the vehicle has moved from a parked position and the
current heading and said fixed direction information differ by a
predetermined amount.
2. The system as defied in claim 1 wherein said fixed direction
information includes at least one averaged value.
3. The system as defined in claim 1 wherein the vehicle includes an
ignition circuit and said processing circuit processes said sensor
signals when said processing circuit determines that the vehicle's
ignition is off.
4. The system as defined in claim 3 and further including a disable
circuit coupled to said processing circuit to provide a disable
signal applied to said processing circuit to prevent the generation
of an alarm output signal.
5. The system as defined in claim 4 wherein said disable circuit
includes means for entering an operator code which matches a code
Stored by said processing circuit.
6. The system as defined in claim 1 and further including an alarm
coupled to said processing circuit and responsive to said alarm
output signal for providing an alarm.
7. The system as defined in claim 1 wherein said magnetic field
sensor is a flux-gate sensor.
8. The system as defined in claim 1 further including an overhead
console for mounting to a roof of a vehicle and wherein said
magnetic field sensor and said processing circuit are mounted to
said overhead console.
9. The system as defined in claim 1 and further including an
electrical compass display coupled to said processing circuit and
wherein said processing circuit is responsive to signals from said
magnetic field sensor when the vehicle is not parked to provide
compass heading information to said display.
10. The system as defined in claim 1 and further including a
trainable garage door opening transmitter.
11. An electronic theft detection system for a vehicle
comprising:
a magnetic field sensor for detecting the earth's magnetic field
and for providing electrical sensor signals representing the
direction of the vehicle in relation to the earth's magnetic field
when parked;
a processing circuit coupled to said sensor for processing said
sensor signals, said processing circuit responsive to said sensor
signals for calculating and storing information representing a
heading of the vehicle when parked, said processing circuit
comparing such stored information with current heading information
and generating an alarm output signal indicating movement of the
vehicle when a predetermined relationship exists between the
current heading and said stored heading information; and
a timer circuit coupled to said processing circuit for supplying an
interrupt signal applied to said processing circuit for causing
said processing circuit to exit a low-power mode of operation and
to enter an active mode of operation.
12. The system as defined in claim 11 further comprising a
non-volatile memory circuit coupled to said processing circuit for
storing information while said processing circuit is in a low-power
mode of operation.
13. The system as defined in claim 12 wherein said timer circuit
periodically supplies said interrupt signal.
14. The system as defined in claim 13 wherein said timer circuit
periodically supplies said interrupt signal about every five
seconds.
15. An electronic theft detection system for a vehicle
comprising:
a magnetic field sensor for detecting the earth's magnetic field
and for providing signals representing a direction of the vehicle
in relation to the earth's magnetic field; and
a processing circuit coupled to said sensor for processing said
sensor signals, said processing circuit responsive to said sensor
signals for calculating and storing a current heading of the
vehicle and the current field strength of the magnetic field
detected by said sensor, said processing circuit comparing the
current heading with at least one stored heading value and the
current field strength with at least one stored field strength
value, said processing circuit generating an output signal
indicating unauthorized movement of the vehicle when the comparison
indicates a predetermined difference in said stored and current
sensor signals, wherein said predetermined difference represents a
difference in heading which is at least four times greater than the
average difference in field strength.
16. The system as defined in claim 15 wherein said at least one
stored heading value and said at least one stored field strength
value each include at least one averaged value.
17. The system as defined in claim 15 wherein said processing
circuit processes said sensor signals when said processing circuit
determines that the vehicle's ignition is off.
18. The system as defined in claim 17 and further including a
disable circuit coupled to said processing circuit to provide a
disable signal applied to said processing circuit to prevent the
generation of an alarm output signal.
19. An electronic theft detection system for a vehicle
comprising:
a magnetic field sensor for detecting the earth's magnetic field
and for providing electrical sensor signals representing a
direction of the vehicle in relation to the earth's magnetic field;
and
a processing circuit coupled to said sensor for processing said
sensor signals, said processing circuit responsive to said sensor
signals for calculating and storing a current heading of the
vehicle and the current field strength of the magnetic field within
the vehicle, said processing circuit calculating an averaged
heading and field strength, said processing circuit calculating an
averaged difference between said current heading and said averaged
heading and an averaged difference between said current field
strength and said averaged field strength, said processing circuit
generating an alarm output signal indicating theft of the vehicle
when said averaged difference in heading exceeds said averaged
difference in field strength by a predetermined amount sufficient
to indicate unauthorized movement of the vehicle.
20. The system as defined in claim 19 wherein said processing
circuit processes said sensor signals when said processing circuit
determines that the vehicle's ignition is off.
21. The system as defined in claim 20 and further including a
disable circuit coupled to said processing circuit to provide a
disable signal applied to said processing circuit to prevent the
generation of an alarm output signal.
22. The system as defined in claim 21 wherein said disable circuit
includes means for entering an operator code which matches a code
stored by said processing circuit.
23. The system as defined in claim 19 wherein said alarm output
signal indicating theft of the vehicle activates the vehicle's
horn.
24. The system as defined in claim 23 wherein the vehicle includes
an engine control and wherein said alarm output signal indicating
theft of the vehicle is applied to said engine control to prevent
operation of the vehicle's engine.
25. An electronic theft detection system for a vehicle
comprising:
a magnetic field sensor for detecting the earth's magnetic field
and for providing electrical sensor signals representing a
direction of the vehicle in relation to the earth's magnetic field;
and
a processing circuit coupled to said sensor for processing said
sensor signals, said processing circuit responsive to said sensor
signals for calculating and storing a current heading of the
vehicle and the current field strength of the magnetic field within
the vehicle, said processing circuit calculating an averaged
heading and field strength, said processing circuit calculating an
averaged difference between said current heading and said averaged
heading and an averaged difference between said current field
strength and said averaged field strength, said processing circuit
generating an alarm output signal indicating theft of the vehicle
when said averaged difference in heading exceeds said averaged
difference in field strength by a predetermined amount, wherein
said predetermined amount comprises an averaged difference in
heading which is at least four times greater than the averaged
difference in field strength.
26. An electronic theft detection and compass system for a vehicle
comprising:
a magnetic field sensor for detecting the earth's magnetic field
and for providing electrical sensor signals representing a
direction of the vehicle in relation to the earth's magnetic
field;
a processing circuit coupled to said sensor for processing said
sensor signals;
a display circuit coupled to said processing circuit;
said processing circuit responsive to said sensor signals for
calculating and storing a current heading of the vehicle and for
generating a display output signal to said display circuit for
displaying the heading of the vehicle when in operation, said
processing circuit comparing the current heading of the vehicle
with at least one stored heading value and generating a theft alarm
output signal indicating theft of the vehicle when a predetermined
relationship exists between the current heading and said at least
one stored heading value when the vehicle is parked; and
a timer circuit coupled to said processing circuit for supplying an
interrupt signal applied to said processing circuit for causing
said processing circuit to exit a low-power mode of operation and
to enter an active mode of operation.
27. The system as defined in claim 26 and further including a
trainable garage door opening transmitter.
Description
BACKGROUND OF THE INVENTION
The present invention relates to theft detection systems and
particularly those for use in vehicles.
Automotive vehicles are an important aspect of life in modern
society. Beyond simply providing a means of transportation,
vehicles today also provide consumers with a variety of optional
features to maximize convenience, comfort, and safety. The trend
towards accesorization, however, has caused automotive vehicles to
become more expensive and thus more valuable. As an unfortunate
result, the threat of automotive theft has become an increasingly
important concern.
A multitude of alarm systems have been designed to prevent or deter
the theft of automobile vehicles. The operation of many of these
systems is dependent upon the detection of activity which is likely
to be attributable to vehicle theft. One method of theft detection
involves sensing unauthorized movement of a vehicle and is
especially useful for preventing vehicle theft accomplished by
towing the vehicle or placing it on a flat-bed track. Alarm systems
usually employ sensors which are dependent upon the motion of the
vehicle causing mechanical movement of internal sensor elements.
One type of sensor senses vehicle motion by monitoring an internal
sensor variable, such as magnetic flux, inductance, or induced
voltage, which changes due to movement of an internal movable
sensor element such as a magnet or a drop of mercury. Mother type
of sensor senses vehicle motion by monitoring an internal sensor
variable, such as capacitance or resistance, which changes due to
movement of special fluids within the sensor.
Although these prior art systems provide a measure of protection
against vehicle theft, their dependency on the motion of the
vehicle causing movement of internal sensor elements may result in
the non-detection of vehicle motion in some circumstances. In
particular, these systems may have difficulty sensing vehicle theft
when the vehicle is initially parked on an incline because the
position of the vehicle may prevent proper motion of the internal
sensor elements. Furthermore, the mechanical nature of the sensors
of most of these prior art theft detection systems increases the
cost of the systems and can reduce their reliability. Also, the
sensors of these systems are application specific such that they
cannot be used to provide functions in addition to theft
detection.
Thus, a need exists for a theft detection system designed to detect
vehicle motion without dependency upon the mechanical movement of
internal system elements such that vehicle theft can be detected in
all situations and that the expense of the system is minimized and
its reliability improved. Furthermore, a theft detection system
having a sensor that is useful in applications in addition to theft
detection would be preferred.
SUMMARY OF THE INVENTION
The present invention provides an improved automotive theft
detection system which detects vehicle theft by sensing
unauthorized movement of a vehicle. The system's operation is not
mechanical in nature such that vehicle theft can be detected in all
situations and the expense of the system is minimized. In the
preferred embodiment, a compass system is also implemented such
that a vehicular feature, in addition to theft detection, is
provided by much of the same electrical circuitry.
The present invention includes a magnetic field sensor for mounting
within a vehicle to provide signal information representing the
earth's magnetic field sensed within the vehicle. The sensor is
coupled to an electrical processing circuit for sampling the data
provided by the sensor and for generating a theft alarm signal when
the heading of the vehicle has changed due to movement of the
vehicle caused by vehicle theft. In the preferred embodiment, the
electrical processing circuit includes a microprocessor programmed
to analyze the sensor data and to use such data to provide a
combined theft detection and compass system.
These and other features, advantages and objects of the present
invention will be further understood and appreciated by those
skilled in the art by reference to the following specification,
claims and appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a fragmentary perspective view of a vehicle embodying the
present invention;
FIG. 2 is an electrical circuit diagram partly in block and
schematic form of the theft detection system embodying the present
invention;
FIG. 3 is a graph illustrating the ideal signal received from the
magnetic field sensor through the interface circuit;
FIG. 4 is an electrical circuit diagram partly in block and
schematic form of the preferred embodiment of the theft detection
system in which a vehicle compass system is provided;
FIG. 5 is a flow diagram of the main program routine of the theft
detection system of the present invention;
FIG. 6 is a flow diagram of the Theft Detection Routine of the
theft detection system of the present invention;
FIG. 7 is an electrical circuit diagram partly in block and
schematic form of an alternate embodiment of the theft detection
system of the present invention; and
FIG. 8 is the flow diagram for the main program routine of the
alternate embodiment of the theft detection system of the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
In FIG. 1, there is shown a vehicle 10 such as an automobile which
includes an overhead console 12 mounted to the roof 14 of the
vehicle during manufacture, although it could be separately added
at a later time. Console 12 is centered near the top edge of
windshield 16 typically above the rearview mirror 18 and includes a
pair of switches 20 for operating lamps positioned behind lenses 22
which in turn direct illumination into the lap area of either the
driver or passenger side of the vehicle depending on which switch
is actuated. The center of the console includes a trainable garage
door opening transmitter 24 of the type disclosed in allowed U.S.
patent application Ser. No. 08/055,509, filed Apr. 30, 1993, and
entitled TRAINABLE TRANSMITTER WITH OUTPUT ATTENUATION CONTROL.
This trainable transmitter can learn the RF frequency, modulation
scheme, and security code of up to three existing remote
transmitters. Thus, console 12 including trainable transmitter 24,
can replace three separate remote control transmitters usually
loosely stored in the vehicle. The transmitter includes three
control switches 26, 28, and 30 and an indicator LED 32 for the
display of training prompting information to the vehicle operator.
Console 12 also includes a display panel 34, the center of which
includes a digital display 36 providing, in one embodiment of the
invention, a sixteen point compass display of the vehicle heading.
Console 12 also includes display control buttons 38, keypad 39, and
the theft detection circuit shown in FIG. 2 which is appropriately
mounted therein.
Referring now to FIG. 2, the theft detection system of the present
invention includes a flux-gate type magnetic field sensor 40
coupled to microprocessor 42 through an electrical interface
circuit 44. Microprocessor 42 and circuit 44 comprise a processing
circuit for processing signals from sensor 40, as described below.
Also coupled to microprocessor 42 in a conventional manner is a
power supply circuit 46, a disable circuit 48, a non-volatile
memory circuit 50, and a timer circuit 52. The functioning and
interconnection of these circuits is now described in more
detail.
In the preferred embodiment of the present invention,
microprocessor 42 is an HC05 eight-bit microprocessor manufactured
by the Motorola Corporation. Operating power is supplied to
microprocessor 42 by means of connection to power supply circuit 46
which is coupled to the vehicle's battery (not shown). Circuit 46
essentially functions as a voltage regulator and converts the
battery supply voltage, typically 12 volts, to five volts which is
supplied to terminal V.sub.DD of microprocessor 42. Circuit 46 also
provides a reset signal to the RESET pin of microprocessor 42
approximately ten milliseconds after the five-volt operating power
is initially supplied to terminal V.sub.DD. The purpose of this
signal is to reset microprocessor 42 so that it begins its
processing cycle at the appropriate time after connection of a
battery to the vehicle.
Microprocessor 42 receives timing signals defining an interrupt for
execution of the Theft Detection Routine as described in connection
with the program flow diagrams. This signal is applied to the
microprocessor via output terminal 55 of a timer circuit 52 which
comprises a resistor 51, a capacitor 53, and a comparator 54.
Capacitor 53 is selectively charged by the output from pin PA0 of
microprocessor 42. When pin PA0 is switched to a high impedance
level, the charge on capacitor 53 drains through resistor 51 with
the rate of discharge proportional to the size of resistor 51. When
the charge on capacitor 53 has sufficiently drained such that the
voltage on the negative input terminal 55 of comparator 54 is less
than a reference voltage (VREF) of one volt maintained at its
positive input terminal by a suitable voltage divider network (not
shown) within supply 46, the output terminal 55 of comparator 54
supplies a logic "1" signal to the INT (interrupt) pin of
microprocessor 42, for reasons discussed below. In the preferred
embodiment, the time for capacitor 53 to discharge such that a
signal is supplied to the INT pin of microprocessor 42 is
approximately five seconds which Corresponds to a value for
resistor 51 of approximately ten megohms.
Non-volatile memory circuit 50 provides an alternate source of
information storage to microprocessor 42. Unlike the typical RAM
memory within the microprocessor 42, memory 50 can maintain its
information storage when power is discontinued. As described below,
microprocessor 42 stores certain variables in circuit 50 before the
microprocessor enters a low-power mode to ensure that the values of
the variables are properly retained.
As described below, the preferred embodiment of the theft detection
system is automatically initiated when the ignition system of a
vehicle is turned off. Disable circuit 48 of FIG. 2 provides to the
vehicle's operator a means by which to disable the system such that
automatic initiation does not occur. This is desirable in many
non-theft situations such as when a vehicle is towed to an
automotive repair shop for repairs. Circuit 48 preferably is a
keypad 39 (FIG. 1) into which a coded input signal is entered and
which provides a disable output signal at terminal 49 which is
coupled to terminal PA6 of microprocessor 42. If the operator's
input disable code matches a predetermined code stored in the
microprocessor's memory, then the theft detection system is
disabled. The theft detection system then remains in a disabled
state until another coded input signal is entered via circuit 48
which matches a predetermined code stored in the memory of
microprocessor 42.
As shown in FIG. 2, microprocessor 42 receives via terminal PA1 an
input signal from the vehicle's ignition system enabling the status
of the system to be determined. As described below, the theft
detection system of the preferred embodiment is operational only
when microprocessor 42 determines that the vehicle's ignition is
off. Some vehicles have the capability of detecting and
differentiating between different phases of the ignition system
cycle such that the above-mentioned determination can be based on
whether an ignition key is in the ignition keyhole. The advantage
of this capability is discussed below. Normally, the ignition
signal is a discrete input signal obtained by means of a direct
connection between microprocessor 42 and the vehicle's ignition
system. However, the ignition signal may also be received via the
vehicle's internal bus system.
Microprocessor 42 is also shown coupled to a theft alarm 41 via
terminal PA5. As discussed below, microprocessor 42 outputs a theft
alarm signal when sensor 44 provides signal information indicating
vehicle theft is being attempted. The theft alarm signal may be
transmitted to a variety of locations, directly or via the
vehicle's internal bus system, to indicate that a vehicle theft is
occurring. Examples include sending the theft alarm signal to the
vehicle's horn to sound an alarm, to the computer or engine control
within the engine to prevent operation thereof, to a pager module
to transmit a message to the vehicle's owner, or to the vehicle's
phone to automatically contact the police. Theft alarm 41
illustrates such connections but also could be a separate alarm
horn, siren, flashing light, or the like to draw attention to the
vehicle and deter the would-be thief.
Sensor 40 is a flux-gate sensor in the preferred embodiment,
although other types of sensors may be employed including
frequency-based capacitative or magneto-resistive or inductive type
sensors. Sensor 40 includes an annular core 56 around which is
wound a primary winding 58, a secondary East/West sensing winding
60, and a secondary North/South sensing winding 62. Primary winding
58 is driven by zero to five-volt signals supplied by terminal 64
of interface circuit 44 to selectively drive annular core 56 into
saturation. Secondary windings 60 and 62 supply signals,
representing the magnetic field sensed within the vehicle along two
axes of measurement, to terminals 68 and 66, respectively, of
interface circuit 44. The windings of sensor 40 are provided an
intermediate reference ground through connection to terminal 70 of
interface circuit 44 which is held at 2.5 volts.
Interface circuit 44 essentially serves as an interface between
sensor 40 and microprocessor 42, performing the same functions as
the corresponding individual circuit components detailed in U.S.
Pat. No. 4,953,305, issued on Aug. 4, 1990, entitled VEHICLE
COMPASS WITH AUTOMATIC CONTINUOUS CALIBRATION, assigned to the
present assignee, and incorporated herein by reference. Interface
circuit 44 preferably is an application specific integrated circuit
(ASIC) essentially incorporating the individual circuits of the
prior interface circuit in a conventional manner to reduce cost.
Interface circuit 44 converts the analog signals from sensing
windings 60 and 62 of sensor 40 into eight-bit digital signals
(count values) which represent the magnetic field strength, in
milligauss, detected by the two channels of sensor measurement.
Each count value represents four to five milligauss of magnetism in
the preferred embodiment of the present invention, although other
conversion ratios may be employed. The digital signals generated by
interface circuit 44 are supplied via terminal 72 to terminal PA4
of microprocessor 42 over bi-directional serial communication line
74.
The digital signals supplied by circuit 44 to microprocessor 42,
representing the magnetic field sensed by the North/South and
East/West channels of sensor 40, can be plotted on an X-Y
coordinate plane, as shown in FIG. 3. The magnetic field strength,
in milligauss, of the East/West channel is represented by the X
axis, and the magnetic field strength, in milligauss, of the
North/South channel is represented by the Y axis. For a properly
calibrated sensor, the plotted channel data creates a perfect
circle around the origin of the coordinate plane when the vehicle
is moved in a 360.degree. loop, as indicated by graph A. The radius
of the circle represents the earth's magnetic field strength, and
the vehicle's compass heading at a particular time is represented
by a point on the circle.
A particular advantage of the theft detection system of the present
invention is its compatibility with a vehicle compass system
providing heading information to the operator of the vehicle. An
example of such a compass system is disclosed in the
above-identified U.S. Pat. No. 4,953,305, the disclosure of which
is incorporated herein by reference. As such, the preferred
embodiment of the present invention provides a combined theft
detection and compass system, the implementation of which is
facilitated since much of the required circuitry is common to both
systems. This enables a substantial cost-savings in the providing
of two distinct and usually separate vehicle accessories. The only
additional circuitry (i.e., hardware) required for the compass
system is a display for displaying heading information to the
vehicle's operator. The combined hardware for such a theft
detection and compass system is shown schematically in FIG. 4, with
a display driver 76 coupled to terminal PA7 of microprocessor 42
and providing signals to the compass display 36 (FIGS. 1 and
4).
The operation of the theft detection system of the preferred
embodiment of the present invention is now described in connection
with FIG. 5 which is the flow diagram for the main program routine
80 for microprocessor 42. The program begins with block 82 which is
the beginning of the main program routine. Next, block 84
determines if the vehicle's ignition system is on by analyzing the
status of pin PA1 of microprocessor 42. As mentioned above, the
theft detection system of the preferred embodiment is operational
only when the ignition system of the vehicle is off. This provides
protection during the typical theft situation when the vehicle is
parked and left unattended. However, the preferred embodiment of
the present invention also implements a compass system, as
described above, when the ignition system of the vehicle is on. As
such, if block 84 determines that the vehicle's ignition system is
on, and if the preferred embodiment of FIG. 4 is incorporated into
the system, the program proceeds to block 86 which executes a
compass routine of a vehicle compass system with microprocessor 42
in an active mode of operation. After block 86, the program loops
back to block 84. If block 84 determines that the vehicle's
ignition system is off, then the program proceeds to block 88.
Block 88 causes microprocessor 42 and interface circuit 44 to enter
a low-power mode in which almost all functioning ceases in order to
avoid draining the vehicle's battery. Power supply 46, however, is
coupled to the vehicle's battery and continues to provide operating
power to microprocessor 44 and circuit 52. In addition,
microprocessor 42 switches port pin PA0 (FIGS. 3 and 4) to a high
impedance level which causes capacitor 53 of timer circuit 52,
which is in a fully charged state when the ignition is initially
turned off; to begin to discharge through resistor 51 such that
timer circuit 52 eventually provides an interrupt signal to
microprocessor 42 as described above. In the preferred embodiment,
the time for capacitor 53 to discharge such that an interrupt
signal is generated is approximately five seconds.
After block 88, block 90 determines if the theft detection system
is in a disabled state by means of operation of the disable circuit
48 connected to terminal PA6 of microprocessor 42. If the system is
disabled, the program proceeds to block 96. If the system is not
disabled, then block 92 determines if timer circuit 52 is providing
an interrupt signal to terminal INT of microprocessor 42. If an
interrupt signal is being provided, then the program proceeds to
block 94 which executes the Theft Detection Routine 99 (FIG. 6).
During execution of this routine, capacitor C1 is fully recharged
so that it can again be discharged after one cycle through the
routine is completed in order to again generate an interrupt
signal. The Theft Detection Routine 99 is described in detail below
in connection with FIG. 6.
After block 94, or if block 92 determines that an interrupt signal
is not being provided, then the program proceeds to block 96. Block
96 determines if the vehicle's ignition system is still off by
analyzing the status of pin PA1 of microprocessor 42. If the
ignition system is still off, then the program loops back to block
90 and the above-mentioned cycle is repeated. As a result, the
Theft Detection Routine is executed approximately every five
seconds in the preferred embodiment of the present invention which
corresponds to the time required for capacitor 53 to discharge such
that an interrupt signal is generated. If block 96 determines that
the ignition system is on, then block 98 causes microprocessor 42
and interface circuit 44 to recover from the low-power mode. The
program then loops back to block 86 to execute the compass routine
of the vehicle compass system while the vehicle's ignition system
is on. A brief description of the Theft Detection Routine now
follows.
The Theft Detection Routine of microprocessor 42 begins by causing
the microprocessor and interface circuit 44 to recover from the
low-power mode. Next, the magnetic field along both the North/South
and East/West sensor channels is determined from the sensor data
provided by interface circuit 44. Analyzing this sensor data in
terms of its position on the X-Y coordinate plane, microprocessor
42 then calculates the vehicle heading and the magnetic field
strength for that five-second period of time (measurement cycle).
Across measurement cycles, microprocessor 42 maintains a running
average for both the heading and field strength. Furthermore, the
running average difference between each running average and the
current heading or field strength is also maintained across
measurement cycles. If microprocessor 42 determines that a
predetermined relationship or difference exists between the average
difference of the heading and the average difference of the field
strength, then microprocessor 42 transmits a theft alarm signal via
pin PA5 indicating that the vehicle is being stolen. In the
preferred embodiment, this predetermined relationship is
established when the average difference of the heading exceeds the
average difference of the field strength by a predetermined amount.
This corresponds to the situation where the sensor data is changing
in a manner such that, over consecutive measurement cycles, the
plot thereof substantially creates an arc on the X-Y coordinate
plane. Such a change in sensor data indicates that the vehicle is
moving and that its heading is changing as would occur when a
stolen vehicle is towed away. The averaging employed in this theft
detection process enables the system to accurately detect vehicle
motion while preventing false detections such as when sensor 40 is
affected by stray magnetic fields located near the vehicle or when
temporarily erroneous sensor data is received. The extent of
averaging can be increased or decreased, or even eliminated,
depending on the amount of sensitivity that is desired. A detailed
description of the Theft Detection Routine programming for
microprocessor 42 to provide this system operation is now provided
in connection with the flowchart of FIG. 6.
In discussing the flow diagram of FIG. 6 for the Theft Detection
Routine 99 of microprocessor 42, the following symbols and their
definitions are used:
X Variable: The East/West digital sensor data.
Y Variable: The North/South digital sensor data.
H Variable: The heading of the vehicle computed from the two axis
magnetic field measurement.
R Variable: The field strength of the external field around the
vehicle computed from the two axis magnetic field measurement.
AH Variable: The running average of H updated every measurement
cycle.
AR Variable: The running average of R updated every measurement
cycle.
DH Variable: The running average of the absolute difference between
AH and H, updated every measurement cycle.
DR Variable: The running average of the absolute difference between
AR and R, updated every measurement cycle.
Referring now to FIG. 6, shown is the Theft Detection Routine 99 of
the programming for microprocessor 42. The routine is executed only
when the vehicle's ignition system is off and after capacitor 53 of
timer circuit 52 has sufficiently discharged such that an interrupt
signal is applied to the INT pin of microprocessor 42. In the
preferred embodiment, component values of timer circuit 52 are
selected such that microprocessor 42 enters the active mode of
operation and the Theft Detection Routine is executed approximately
every five seconds when the vehicle's ignition is off. This
corresponds to an average ignition off current draw through
microprocessor 42 of less than 1 milliamp which is a low current
draw to prevent the draining of the vehicle's battery.
Block 100 of FIG. 6 is the beginning of the Theft Detection
Routine. After block 100, the program proceeds to block 102 which
causes microprocessor 42 to recover from the low-power or "STOP"
mode such that normal functioning of the microprocessor is
initiated. Next, block 104 initializes the magnetic sensor circuit,
comprising sensor 40 and interface circuit 44, terminates the high
impedance level status of terminal PA0 of microprocessor 42, and
begins charging capacitor 53 of timer circuit 52 via terminal PA0.
Initialization of the magnetic sensor circuit involves supplying a
signal from terminal PA4 of microprocessor 42 to interface circuit
44 instructing it to recover from its low-power mode, and waiting
until circuit 44 has had an opportunity to stabilize. Charging of
capacitor 53 is initiated so that it is fully charged by the time
the execution of the Theft Detection Routine has been completed
such that it can be subsequently discharged. To ensure that the
execution of the Theft Detection Routine is of sufficient duration
such that capacitor 53 is fully charged, delay loops (not shown)
can be implemented in the programming in a conventional fashion.
After block 104, the program proceeds to block 106 which collects
eight-bit digital data from interface circuit 44 representing the
magnetic field sensed on the North/South (Y axis) and East/West (X
axis) channels of sensor 40. Next, block 108 calculates the heading
H of the vehicle in a conventional manner using the following
formula: ##EQU1## The program then proceeds to block 110 which
calculates the magnetic field strength R of the field external to
the vehicle using the following formula:
The magnetic field strength R is represented by the radius of the
circle plotted by the sensor data when the vehicle is moved in a
360.degree. loop. Next, block 112 calculates the average of the
vehicle heading H and stores it in variable AH using the following
formula:
The multiplication factors used in this formula are designed to
give the previously stored average heading more numerical weight
than the current heading, and can be adjusted depending on the
desired rate of change of the average heading. Because an averaged
heading value is not available when the ignition is first turned
off, variable AH is set to the current instantaneous heading H upon
first pass through the Theft Detection Routine. After block 112,
the program proceeds to block 114.
Block 114 calculates the average of the field strength R and stores
it in variable AR using the following formula:
Similar to the above, the multiplication factors used in this
formula are designed to give the previously stored average field
strength more numerical weight than the current field strength, and
can be adjusted depending on the desired rate of change of the
average field strength. Because an averaged field strength value is
not available when the ignition is first turned off, variable AR is
set to the current instantaneous field strength R upon first pass
through the Theft Detection Routine. Next, block 116 calculates the
average heading difference and stores it as variable DH using the
following formula:
Similar to the above, the multiplication factors used in this
formula are designed to give the previously stored average heading
difference more numerical weight than the current heading
difference, and can be adjusted depending on the desired rate of
change of the average heading difference. Variable DH is
initialized to zero upon first pass through the Theft Detection
Routine. After block 116, the program proceeds to block 118 which
calculates the average field strength difference and stores it in
variable DR using the following formula:
As above, the multiplication factors used in this formula are
designed to give the previously stored average field strength
difference more numerical weight than the current field strength
difference, and can be adjusted depending on the desired rate of
change of the average field strength difference. Variable DR is
initialized to zero upon first pass through the Theft Detection
Routine. After block 118, the program proceeds to block 120.
Block 120 determines if the current average heading difference
exceeds the current average field strength difference by a
predetermined amount. In the preferred embodiment, this
predetermined mount is exceeded when the average heading difference
is more than four times larger than the average difference of the
field strength. If block 120 determines that the predetermined
amount has been exceeded, then the program proceeds to block 122
which causes microprocessor 42 to signal via terminal PA5 that the
vehicle is being stolen. After block 122, or if block 120
determines that the predetermined amount has not been exceeded, the
program proceeds to block 124. Block 124 stores the current values
of variables AH, AR, DH, and DR in non-volatile memory 50 so that
they are maintained when microprocessor 42 subsequently enters the
low-power mode. Next, block 126 causes microprocessor 42 and
interface circuit 44 to reenter the low-power mode in order to
conserve energy, and switches port pin PA0 to a high impedance
level so capacitor 53 of timer circuit 52, having been fully
charged, begins to discharge. The program then exits the Theft
Detection Routine via block 128.
Although the preferred embodiment of the present invention detects
theft of a vehicle by analyzing and comparing both heading and
field strength information, an alternate embodiment can function by
monitoring only the heading of the vehicle. Microprocessor 42 in
such an embodiment can detect vehicle theft when the averaged
difference of the heading exceeds a predetermined value or when a
predetermined relationship exists between the current heading and
at least one stored heading value. The only disadvantage of this
embodiment is that a change in heading caused by outside influences
other than movement of a vehicle can potentially cause an erroneous
detection of vehicle theft. This problem is overcome in the
preferred embodiment described above through comparison of the
heading information to the averaged difference in field strength to
ensure that the plot of sensor data substantially forms an arc
which corresponds to movement of the vehicle.
As described above, the preferred embodiment of the theft detection
system is operational only when a vehicle's ignition system is off.
The advantage of such an embodiment is that it is a convenient
means by which to provide a passive theft protection system that is
automatically activated when the ignition is turned off without
requiring a specific initiation step by the vehicle's operator.
However, if the vehicle's ignition system is hot-wired after
unauthorized entry, then the theft detection system may be
deactivated depending on what is analyzed by microprocessor 42 via
pin PA1 to determine the status of the ignition system. As such,
theft protection may only be provided in the situation where the
vehicle is stolen via towing or placement on a flat-bed truck. Some
vehicles, though, may have the capability of detecting and
differentiating between different phases of the ignition system
cycle such that this disadvantage can be avoided. In particular,
these vehicles can detect placement of an ignition key in the
vehicle's ignition keyhole and supply this information to
microprocessor 42 via pin PA1. As such, the theft detection systems
of these vehicles can be programmed to deactivate only upon
detection of that event so that the system remains armed if the
vehicle's ignition is hot-wired.
An alternative embodiment of the present invention enables theft
protection to be provided independent of the status of the
vehicle's ignition. As shown in FIG. 7, this alternate embodiment
includes an operator input circuit comprising, in the preferred
embodiment, a transmitter located on a key FOB 130 and a receiver
circuit 132 connected to microprocessor 42. Briefly described,
activation of the theft detection system by the vehicle's operator
may be achieved by means of transmitting a coded RF input signal
from key FOB 130 to receiver circuit 132 before the vehicle is left
unattended. The system then remains active until a second coded RF
input signal is again transmitted via key FOB 130 and received by
receiver circuit 132. The advantage of this embodiment is that
theft protection can be provided even when the ignition system is
on such that theft by means of hot-wiring the vehicle can be
detected. In order to provide passive theft protection in case the
vehicle's operator neglects to initiate an activation signal, the
theft detection system may also be programmed to automatically
activate when the ignition is turned off, as described above. A
more detailed description of the operation of this system is now
provided in connection with FIG. 8 which is the flow diagram for
the main program routine 134 of the alternate embodiment of FIG.
7.
The main program routine 134 of the alternate embodiment is the
same as the main program routine 80 described above except for the
addition of blocks 136, 138, 140, 142, 144, and 146. After block 82
which is the beginning of the main program routine, block 136
determines if a coded activation input signal from FOB 130 has been
received by receiver 132 which matches a predetermined code stored
in the memory of microprocessor 42. If such a signal has been
received, then the program proceeds to block 138 which causes
microprocessor 42 and interface circuit 44 to enter the low-power
mode, and switches port pin PA0 to a high impedance level so that
capacitor 53 can begin to discharge. After block 138, the program
proceeds to block 140 which determines if timer circuit 52 is
providing an interrupt signal to terminal INT of microprocessor 42.
If an interrupt signal is being provided, then the program proceeds
to block 142 which executes the Theft Detection Routine described
above in connection with FIG. 6. After block 142, or if block 140
determines that an interrupt signal is not being provided, then the
program proceeds to block 144. Block 144 determines if a coded
deactivation input signal has been received via receiver 132, which
matches a predetermined code stored in the memory of microprocessor
42, to deactivate the theft detection system. If not, the program
loops back to block 140 and the above-mentioned process repeats
itself. If a deactivation input signal has been received, then the
program proceeds to block 98 which causes microprocessor 42 and
interface circuit 44 to enter the active mode of operation so that
the compass routine of block 86 can be executed.
Returning to block 136, if an activation input signal has not been
received, and the theft detection system is automatically
implemented because the vehicle's ignition system is off, then the
operation of the system proceeds as described above in connection
with FIG. 5 except for the addition of block 146. If block 146
determines that a coded activation input signal has been received
via the operator input circuit (i.e., receiver 132) which matches a
predetermined code stored in the memory of microprocessor 42, then
the program branches to block 140. This enables the theft detection
system to remain activated, even after the ignition system is
turned on, until a coded deactivation input signal is received.
The present invention provides an improved and relatively
inexpensive electronic theft detection system that can detect
vehicle theft in all situations. Furthermore, a theft detection
system is provided having circuitry that is compatible with a
vehicle compass .system such that a combined theft detection and
compass system can be conveniently implemented in the preferred
embodiment.
The above description is considered that of the preferred
embodiments only. Modifications of the invention will occur to
those skilled in the art and to those who make or use the
invention. Therefore, it is understood that the embodiments shown
in the drawings and described above are for illustrative purposes
and are not intended to limit the scope of protection of the
invention, which is defined by the following claims as interpreted
according to the principles of patent law.
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